CONCRETE COMPOSITES A NEW CONSTRUCTION MATERIAL

INTRODUCTION/HISTORICAL DEVELOPMENT

The basic ingredients in concrete are aggregates, cement and water. As concrete technology developed, adjustments in the design ∎ix were perfected to improve the ultimate strength of this material. This premise is still the goal in concrete research today. The performance of concrete is dictated by two variables. The first is the specific application for the material and the second variable is the environment where the material will be placed and utilized.

Historical records from the Unversity of Wisconsin reveal that both variables were researched simultaneously. Through trial and error it was determined that extreme temperatures affected the long term performance of concretes.

Research testing proved that concrete should not freeze before reaching initial set or be exposed to extreme heat during this hydration period. The technology of the time led to the use of salts and sugar in the wet concrete. From this concept came the eventual use of calcium chloride in the cement matrix. This was the start of what is know known as admixtures in concrete. The broad term admixture now encompasses additives which are in the form of liquid, powder and even slurry as they are mixed into the concrete. Admixtures range from retarders, accelerators to superplasticers.

The second variable was the performance of the concrete. As steel became plentiful and cost effective it became the primary reinforcement in concrete. The concept of mechanical bonding between concrete and steel was discovered late in the last century. Many configurations of steel size and shape were tried to improve the mechanical bonding performance of the steel to the concrete.

According to Ramachandran et al. [1981], a review of the U.S. Patent Office during the 1920s revealed several applications for steel fibers to be used as a reinforcing material in concrete. This was the birth of fiber reinforced concrete (FRC). Steel fiber reinforce concrete was finally tested for field performance in the early 1970s. According to Yrjanson and Halm [1973], three lanes of the Tampa, Florida International Airport were paved with steel fiber reinforced concrete overlay. In late 1972, the most ambitious experimental fibrillated research project was completed in Greene County, near Jefferson, Iowa. The county engineer reported there were 3.3 miles of fibrous concrete placed as part of a research paving project.

Many types of synthetic fibers have been tested. The range has been wide and extensive. The Soviet Union tested E-glass fibers with no long term success. Nylon and rayon were tested but did not meet the performance characteristics required for concrete reinforcement.

Polypropylene fibers (PPF) have been used in concrete since 1965. Golfein [1965] suggested to

the U.S. Army Corp of Engineers the inclusion of polypropylene as an admixture in concrete for use in blast-resistant buildings. It should be noted that a British patent was registered in 1968. The patented product was given the manufacturer name of Caricrete.

Also during the 1920s welded wire fabric (WWF) or more commonly called steel mesh, began to appear as an experimental reinforcement in concrete slabs. It was not until the building boom of the late 1940s that WWF become the largest single secondary reinforcing material in concrete. Welded wire fabric has been specified for slab on grade use for nearly 40 years. It is the opinion of some concrete specialists and contractors in the construction industry that little structural value is gained from the use of the light gage WWF. What is interesting to note however, is that light weight WWF (.135 diameter steel) was not performance tested for many years. It was specified as a secondary reinforcing material but a paucity of research data was not available until its secondary reinforcement value was compared to polypropylene fibers.

MATERIAL PERFORMANCE CHARACTERISTICS

Steel fibers

Steel fibers are manufactured from drawn wire. The benefits and liabilities are much the same as primary reinforcing steel. Steel fibers are subjected to surface deformation to improve the mechanical locking with the concrete, the same as primary reinforcing steel. Steel fibers add strength to the concrete matrix just like primary reinforcing. There is an optimum volume of steel fiber to improve the performance however.

This volume fraction term is called the aspect ratio and refers to the steel fiber length divided by its diameter. Hannant [1978] presents and equation which is used to estimate the approximate amount of steel fiber for aggregates of normal density.

The equation is:

The equation is designed to be concerned with only larger aggregate because sand and small aggregate do not effect the workability of the concrete when using fibers in concrete.

Field results indicate that steel fiber may best be added by weight in a range from 3 to 7%. Some field reports indicate fiber volume up to 400 pounds per cubic yard. This extreme volume fraction requires an admixture such as a superplasticizer. There is a host of research reports published relating to the positive effect of steel fibrillated concrete. Conversely, there are also draw backs to the use of this product. Steel fibers are bulky and require special care during the mixing cycle. It is also labor intensive to add steel fibers during the mixing cycle. If the fibers are not mixed properly, the concrete will form into balls. Referred to as balling, the coarse aggregates and steel fibers form large balls during the mixing cycle and produce an unacceptable final product.

Fiber reinforced concrete (FRC) has been proven to be a crack arrester. FRC does eliminate most micro cracking and assists in preventing plastic shrinkage. Figure 1 graphically presents the process for stopping crack propagation.

Also pictured in this figure is the random orientation of the fibers as they lie between the coarse and fine aggregates. This three dimensional strength enhancement has proven very effective when comparing concrete tests in tension and flexure.

There is no doubt from field applications that steel fibers enhance the secondary performance of concrete. With the use of many admixtures in concrete, care must be taken to be sure the steel fibers are not affected by the admixture chemical composition. One example is calcium chloride, which is a common admixture in concrete. This salt base admixture attacks the steel fibers and causes corrosion to form around the fibers. Edgington [1973] has presented evidence that over a period of time, these fibers fail structurally. As cracks appear in the concrete with age, moisture works into these cracks. Salt erosion from highways or salt water are just a few examples. Rust bleeding in architectural concrete is another application where damage must be considered.

Polypropylene fibers

Polypropylene fibers (PPF) are utilized as a secondary reinforcement and are manufactured in the isotactic configuration. They are extruded through a flat die and then slit into tape form. The tape is next monoaxially stretched; this process is referred to as "draw ratio." The draw ratio is a measure of the extension which is applied to the fiber during fabrication, and draw ratios of about 8 (eight times its original length) are common for polypropylene film. A molecular orientation results from the stretching process and the resul is high tensile strength. It may be noted, PPF is at its strongest state when placed in concrete with tension or flexure as the primary design criteria.

The raw material, polypropylene, derived from the monomeric C2 H6, is a pure hydrocarbon. According to Zonsvield [1976], its mode of polymerization, its highly molecular weight, and the way it is processed into fibers combine to give PPF many useful properties.

PPF has a sterically regular atomic arrangement in the polymer molecule with high crystallinity. Its regular structure gives it the name isotactic polypropylene. The material has a high melting point of 165 degrees C and has the ability to withstand temperatures over 100 degrees C for short periods of time. This temperature durability is important as secondary reinforcing material in concrete. Polypropylene is chemically inert, which makes the fibers resistant to most chemicals. If the concrete is exposed to aggressive chemicals, the cement will be the first material to deteriorate.

Since PPF has a hydrophobic surface, no additional water is needed in the concrete mix. Because of this slick surface there is less chance for balling during the mixing process as compared to steel fibers. Stated in textile terms, its capabilities are 5g/denier, which is equivalent to 400 MN/m squared. Zonveld [1976] goes on to state that this orientation leaves the film weak in the lateral direction. This opening allows for the wet cement matrix to wrap around the fibers during mixing, which in turn forms a mechanical bonding with the concrete matrix.

PPF has an advantage of being light in weight. Major North American PPF suppliers specify its use at approximately one and a half pounds per cubic yard or about .01% by volume per cubic yard of This is about 1.5 pounds of fiber per cubic yard of concrete. Generally steel fibers will be added at 100 pounds per cubic yard of concrete.

There is no special mixing requirement necessary for PPF when mixed in the traditional over-the-road concrete nixing truck. Forte Corporation (one major supplier), specifies that PPF can be added at the batch plant or on-the-job site prior to the placing of the concrete in its forms. One major concrete supplier in the Midwest reported that adding the polypropylene fibers with the coarse aggregates caused the fibers to open up sooner and resulted in a more homogeneous fiber distribution. This firm also claimed that this extra step eliminated all balling problems which resulted in delivery of a consistent high quality concrete for contractors.

Just as every material has useful applications, when utilized in a large variety of settings, shortcomings do exist with polypropylene fibers. Even though PPF has a high melting temperature compared to other polymers, it does not withstand heat from fires in structures. When exposed to extremely high temperatures, which is common in fires, PPF vaporizes and leaves a void in the concrete equal to the volume of the original PPf in the mixture. This void left after a fire also creates a porosity condition in the concrete which can not be corrected.

Since PPF has a low modulus of elasticity, a high strain rate occurs before multiple cracking appears. When compared to steel, which has higher modulus of elasticity, PPF does not react in the same manner as steel. Between steel and concrete, there is a close coefficient of expansion and contraction. Once a crack occurs in concrete, the steel reinforcing assumes the total load for the concrete mass. This creates a safety net, so to speak. PPF and concrete react very differently. Because of its low modulus of elasticity, the polypropylene fibers, directly assume the load until pullout occurs along the surface of the fibers. At this point, known as the modulus of rupture (NOR), ultimate failure occurs. In some testing, NOR in PPF is higher than in secondary steel reinforcing. It should be recognized that there is a larger coefficient of expansion and contraction between PPF and concrete than there is between steel and concrete. This accounts for some of the differences in the behavior of this material.

In Figure 2 is a drawing which depicts the classification of fiber arrangements. As can be viewed in specimen 1D, the fibers are aligned in a single plane, are parallel to the longest side, and are centered in the object. A matting arrangement is presented in specimen 2D. This is a newer configuration of PPF, which has been recently field tested. In the cube marked 3D, or three dimensional, is a specimen which shows how PPF is mixed throughout the concrete matrix. When random fibers are mixed throughout the concrete, a three directional reinforcing matrix develops. It should be understood that steel fibers are linear in this three directional reinforcing. Polypropylene fibers, being flexible, will wrap themselves around the coarse aggregates in a some what different configuration.

Chemical admixtures

The two chemical admixtures chosen for investigation in this study were experimental additives. The process used to introduce these admixtures into the concrete was also an experimental procedure. The first chemical admixture tested was oleic acid. This is a natural chemical which is obtained by the hydrolysis of vegetable fats, primarily from olive oil. It is separated from the olive oil by the double fractionation urea aducts process. The chemical makeup is C 76.54%, H 12.13%, and 0 11.33%. Because this chemical is an experimental admixture the performance of this chemical will be explained in detail the discussion section.

Basic-H is a commercially produced solution supplied at full strength as a 28% water solution base. The chemical is a nonionic surfactant of linear alcohol alkoxylates. The manufacturer specifies that this agent when used as an admixture will improve the strength of the concrete.

EXPERIMENTAL DESIGN AND ANALYSIS

The results reported in this paper were selected from two separate investigations. One covers results from the chemical admixture composite group and the second reports results of composites in relationship to secondary fiber reinforcement only. The experimental designs for both projects are the same. The testing procedures that both investigations followed conformed to standard concrete practices of The American Society for Tests and Measurements (ASTM) and the American Concrete Institute (ACI).

Both experiments are factorial designs. The cast specimens were mechanically tested and the data was evaluated using the statistical analytical system. Each project was designed to compare samples from the control group with samples in the treatment group. All groups are fully randomized, replicated and verifiable statistically.

Chemical treatment investigation

As reported earlier the two chemicals tested in this project were oleic acid and Basic-H. The process chosen to inject the chemical admixture into the concrete was to coat the polypropylene fiber surfaces with the admixture. Each set of fiber groups were allowed to soak in a designed chemical bath for 10 minutes and then allowed to air dry. At a later time the surface treated PPF were introduced into the concrete while it was being mixed.

As can be seen on Figure 3, there is a control group concrete with no fiber reinforcement. The three treatment groups are listed as plain polypropylene fibers (no chemical admixture surface treatment) polypropylene fibers with oleic acid on the surface and polypropylene fibers with Basic-H on the surface.

Results from the flexure test indicate the Basic-H admixture treatment group achieved the highest strength values after 28 days of curing time. These results are the averages of 14 test beams in each group with a total of 56 beams overall. All flexure tests were conducted following ASTM procedures conforming to C-78 specifications.

Presented in Figure 4, are the tensile test results after 28 days curing period. In this test there were a total of 6 samples tested in each group. In all testing, each group was split and replicated to improve the reliability of the research and statistical results.

Composite fiber investigation

This second part of the investigation reports concrete enhancement through the use of composite fiber reinforcement, since steel and polypropylene fibers have been tested extensively individually.

Reported in Figure 5 are the results of this composite. This test was conducted with a total of 24 flexural beams. The averages are reported on this bar chart. To date these two materials have been tested with commercial admixtures. Dr. Naamen at Michigan State and other researchers have reported their findings.

PRESENTATION OF DATA

The mechanical testing results of the concrete specimens are listed on the figures. The flexure test was conducted with two measures for each sample. A direct measure was calculated for the elastic limit of the beam at center point, more commonly referred to as "E". The second measurement calculated was the modulus of rupture (MOR). The modulus of rupture is calculated from the relationship

The modulus of rupture of the strongest treatment group was the Basic-H enhanced fiber treatment. Conversely the weakest set was the oleic acid group. The composite fiber tensile test revealed the steel fiber group reached the highest applied load at first crack failure of the concrete. The tensile test was conducted utilizing the tensile briquette technique. Since the cross sectional area of each specimen is one square inch, a direct conversion is calculated from the applied load.

DISCUSSION

Mechanical testing of concrete will generate data to analyze and reveal a statistical inference from these results. There are other additional methods of analysis to assist in explaining these test results. One tool available is the use of nondestructive evaluation. The scanning electron microscope (SEM) is a powerful tool to assist in materials research. A brief summary of the scanning electron microscope findings is included to help explain the results of the plain concrete, concrete with plain polypropylene, and PPF treated with oleic acid or Basic-H. As can be viewed in Figure 4, there is a statistical significance between plain concrete and PPF with Basic-H.

Chemical fiber treatment

The data reported here are the first results of several investigations using oleic acid and Basic-H as an admixture. The reader can see that the findings presented in Figures three and four do not clearly indicate which chemical treatment had the highest and consistent performance enhancement in the concrete. One of the goals at the beginning of this research was an attempt to improve the interfacial bond between the polypropylene fiber and concrete matrix.

The oleic acid SEM micrographs revealed a crystalline matrix at the fiber interface. There was no doubt that an interfacial chemical enhancement was achieved. Why had this bonding not improved the overall ultimate strength of the concrete? Not until careful examination of the SEM photos was it discovered that the crystalline growth had a lower elastic modulus than the polypropylene fibers. What occurred was a failure at the fiber interface or a sleeve slipping action. Therefore lower ultimate strengths were recorded in the oleic acid samples. This explains the poor performance of these fibers in tension. An extensive review of this phenomenon can be viewed in work by Lovata and Fahmy [1987].

The results of Basic-H as an a full scale laboratory admixture are not conclusive. The test results from several investigations indicate potential use of this chemical as an early high strength concrete enhancer. It can be reported that long term testing (45 day tests) of Basic-H had not revealed a statistical significant improvement in the concretes ultimate strength.

Composite fiber treatment

The fiber flexure test reveals the averages at first failure in the concrete. As observed in Figure 5, there is a modest gain in the performance of the concrete with polypropylene as a secondary reinforcement. From this one observation (at first crack observation) it appears the steel fiber reinforced concrete has the highest potential for ultimate strength. It should be understood that in the stress strain relationship where concrete is subjected to tension or flexure the concrete will crack. What happens after that initial failure is referred to as the post peak loading condition.

The same concrete failure occurs whether it is in a laboratory situation or field application. Once the concrete cracks, the primary reinforcement, steel re-bars and secondary reinforcement, fibers assume the load for the structure. Therefore the long term performance of the concrete or post fracture condition must be considered. Shah et. al.[1988] has reported findings regarding this condition. The performance results from the composite fiber concrete reveal a change in the elastic limit "E" value of the concrete. After the first crack was recorded in the composite concrete, the post-peak loading curve indicated a change in performance compared to the steel fiber reinforced concrete. The composite concrete sustained (by time) a much higher load curve.

Once the composite post-peak loading situation is fully investigated and reported, it will no doubt will have a long term effect on the future design of composite fiber reinforced concrete.

CONCLUSION

This investigation verifies that experimental chemicals and composite fibers can improve the overall secondary reinforcement performance of the concrete matrix.

ACKNOWLEDGMENT

This research was sponsored in part by Forta Fiber Corporation, Grove City, PA.

REFERENCES